For position detection of moving machine parts and drives resolvers have proven to be reliable and cost-efficient in industrial applications. If the controlling of the machine or the application requires a determination of an angle very frequently resolvers are used due to their robust construction. The resolver is an absolute position or angle measurement system that is based on an inductive principle and is in its configuration similar to an electric motor having precision windings. While the stator of the resolver bears two winding groups whose winding planes are perpendicular and are spatially offset to each other (Wsin, Wcos), the rotor comprises a rotary transformer that is supplied at its primary side by an excitation winding (WRef) and that is inductively coupled with its secondary side with the stator windings.
According to the principle of operation a resolver is configured such that at the input side an excitation with a carrier frequency occurs and such that due to the resolver motion this signal is amplitude-modulated and is transformed to the output side. Due to the orthogonal arrangement of the windings the amplitude-modulated output signal have a phase shift of 90° to each other and are available for further analysis. There are several analysis methods for resolvers.
The application of a resolver may be diverse. Sometimes there is measured and integrated the angular speed and not the rotation angle. It is, however, essential, that one is able to determine, either directly or indirectly, for example by integration, the angular position of the rotor with respect to the stator. The angular information or the rotation speed information is supplied to a frequency inverter or to any other electronic motor controller and is used therein for controlling the rotation speed and/or the torque.
When the resolver fails for some reason the electronic motor controller does not receive a signal. A failure is to be understood as any state in which the correct rotation angle signal is not received by the motor controller. This can be, for instance, an interruption of the signal line between the resolver and the motor controller. In case that the motor controller does not receive the correct signal, this will result in a wrong behaviour during operation. For example, it may be that a motor controller upon using a current vector control regime with rotation speed feed back as is usual nowadays adjusts the rotation speed of the motor to its maximum, when the signal line is interrupted. It is evident that such operating states are not desirable.
In the prior art several devices and methods are known in order to identify such failure situations in the context of rotational angle sensors so as to subsequently take measures for error correction or at least for error treatment. For example the rotational angle sensor may be configured in a redundant manner, that is, a second rotational angle sensor may be provided and the results of both rotational angle sensors may be compared with each other. Moreover, the operation of the rotational angle sensor may be monitored by means of a built-in control electronic.
Furthermore, it is possible to detect a wire breakage in the conductors to and from the resolver that is used in electric machines for angle determination.
In order to identify wire breakage-based failures during the determination of angles typically the input and output signals of the resolver Wsin, Wcos, WRef are electrically fed back and the current flow in these windings is analysed. In this manner the conductor lines to and from the resolver including the windings connected thereto are monitored. A direct monitoring of the windings of the rotary transformer in the rotor is, however, not possible in this manner. A monitoring of the winding can only be accomplished indirectly and in this case the used resolver analysis procedure has an essential influence on the applicable options.
A usual resolver analysis method is the so-called backwards procedure in which the orthogonal windings of the resolver as Wsin and Wcos are supplied with current such that no voltage is induced in the excitation winding as WRef. If the resolver rotor is rotated in this state of signal equilibrium, then this equilibrium state is disturbed and a voltage is transferred into the winding WRef. In this procedure this voltage is supplied to a controller (the so-called tracking controller), which corrects the voltage amplitudes at the orthogonal coils such that the voltage across the excitation winding again becomes zero Volt and the signal equilibrium is re-established. Since the rotor of the resolver is a rotary transformer having respective windings, naturally only AC signals can be transmitted by the transformer. For this reason the input signals of the resolver are amplitude-modulated carrier frequency signals.
The complete monitoring of a system that is based on the above described tracking control principle is difficult. In such a system in the “normal state”, i.e., when the tracking control loop is adjusted, about zero Volt are measured at the input amplifier. The same voltage amplitude, however, is also present when a wire breakage occurs in the windings of the rotary transformer of the resolver. Therefore, errors within the resolver are not detectable.
One possible approach for detecting such an error is the usage of a so-called test mismatch angle. In this manner the tracking control loop does not receive the target value being equal to zero Volt, but being not equal to zero. In such a system the signal at the input amplifier is also unequal to zero in the adjusted state and a wire breakage would thus be detectable.
Typically, the input amplifier is, however, dimensioned such that even low mismatch angles that already occur during, for instance, dynamic events, will saturate the input amplifier. This high amplification is necessary, since only with this measure the control loop is able to follow highly dynamic motions
Measurements revealed that the test mismatch angle has to be in the order of magnitude of about 1° in order to be unambiguously detectable. Unfortunately, this fact and the required high amplification of the input amplifier cause an evidently asymmetric modulation of the input amplifier, thereby negatively affecting the control loop stability.
This known technique may thus not always be applied.